An MRI apparatus is a medical image diagnostic apparatus that causes nuclear magnetic resonance to atomic nuclei in an arbitrary cross section across an object to acquire a tomographic image in the cross section from nuclear magnetic resonance signals to be generated. A radio frequency (hereinafter, referred to as RF) wave that is a type of electromagnetic wave is transmitted to the object to excite spins of the atomic nuclei in the object, and then the nuclear magnetic resonance signals to be generated by the atomic spins are received to generate an image of the object. The RF transmission to the object is performed by an RF transmission coil, and the nuclear magnetic resonance signal reception from the object is performed by an RF reception coil.
A static magnetic field strength tends to be larger in order to improve an SNR (Signal to Noise Ratio) of an image, and a high magnetic field MRI apparatus (ultra-high magnetic field MRI apparatus) whose static magnetic field strength is equal to or more than 3T (Tesla) has prevailed recently. However, the larger the static magnetic field strength becomes, the more the SNR is improved, which can easily cause unevenness in the generated image. This is because an RF frequency to be used for exciting a nuclear magnetic resonance phenomenon is increased by a higher magnetic field.
For example, a 128-MHz RF is used for an MRI apparatus whose static magnetic field strength is equal to or more than 3T (Tesla) (hereinafter, referred to as a 3T MRI apparatus). The RF wavelength is approximately 30 cm that is almost the same scale as an abdominal cross section in the biological body, and a change occurs in the phase. An inhomogeneous spatial distribution is generated in a rotating magnetic field (hereinafter, referred to as a high-frequency magnetic field: B1) generated by an RF and a nuclear magnetic resonance phenomenon excited by the RF due to the change in the phase, which causes image unevenness. Therefore, a technique to reduce the inhomogeneity of the spatial distribution in the high-frequency magnetic field B1 is required for RF irradiation to be executed in an ultra-high magnetic field MRI apparatus.
An RF irradiation method referred to as “RF shimming” is used to reduce inhomogeneity of a B1 distribution. This method reduces B1 inhomogeneity in an imaging region by using an RF transmission coil having a plurality of channels to control a phase and an amplitude of an RF signal to be provided to each channel (for example, refer to Patent Literature 1). A B1 distribution of each channel is measured previously before main imaging, and the amplitude and the phase of an RF pulse appropriate for each of the channels are calculated in order to reduce the B1 inhomogeneity using the B1 distribution. At this time, a region that is a part of a cross section and should be diagnosed is set as a region of interest (ROI), and the amplitude and the phase of the RF pulse for each of the channels are determined so as to reduce the B1 inhomogeneity in the ROI (for example, refer to Patent Literature 2).